Had a problem with this for years.
Forgive my "layman's" jargon.
Previous dialogues with site Admin & Tech personnel tried to locate a "problem" with my modem. That ceased when exactly the same "problem" arose with a different ADSL2 modem.
Was a problem then with my broadband connection, which used a shared, not dedicated, address at the exchange.
However, the problem was *only* with remote servers run on Microsoft Windows and was dated to a server upgrade ~3.5 years ago.
Other remote servers e.g. Unix didn't care
Once I changed providers, the problem disappeared altogether.
It seems to be a connection speed? issue. I have blown my 2 gigabyte allocation and the transfer speed slows to around 7 kbs.
Logging on to the remote Microsoft Windows FTP server I got no timeout message but:
Could not retrieve directory listing -Can't open data connection
I deselected passive mode and removed permanent caching and, bingo, got in.
Hope this helps,